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Abstract:

Honeycomb catalyst structures and methods of using them, where the
structures have honeycomb channel walls of selective catalytic reduction
catalyst, the channel walls occupy at least 20% of the volume of the
structure, the structure exhibits a pressure drop for flowing air not
exceeding about 110 Pa at a space velocity of 20,000 hr-1, and the
channel walls are of a thickness insuring high degree of catalyst
utilization and NOx conversion efficiency.

Claims:

1. A honeycomb structure having channel walls consisting essentially of a
selective catalytic reduction catalyst, wherein the channel walls occupy
at least 20% of the volume of the structure and the structure exhibits a
pressure drop for flowing air not exceeding 110 Pa at a space velocity of
20,000 hr.sup.-1.

2. The honeycomb structure of claim 1 having a channel wall thickness not
exceeding 250 microns.

5. The honeycomb structure of claim 1 providing a nitrogen oxide
conversion efficiency of at least 45% when processing a gas mixture
comprising 500 ppm (volume) of ammonia and 500 ppm (volume) of nitrogen
oxide (NO) in air at a space velocity of 20,000 hr-1 and a gas
mixture temperature of 250.degree. C.

6. The honeycomb structure of claim 5 having honeycomb channel length of
at least 15 cm and a catalyst utilization factor of at least 80%.

7. The honeycomb structure of claim 5 having a cell density of at least
350 channels per square inch of a transverse honeycomb cross-section.

8. The honeycomb structure of claim 7 having a cell density of from about
350 to about 600 channels per square inch of transverse honeycomb
cross-section and a channel wall thickness of about 100 to about 250
microns.

9. A honeycomb catalyst structure comprising channel walls composed of a
selective catalytic reduction catalyst, said catalyst comprising a
dispersion of a pure catalyst within a solid matrix material for binding
the pure catalyst into said walls, the structure having a cell density in
the range of 350-600 cells/in2 and a channel wall thickness in the range
of 100-250 microns.

10-16. (canceled)

17. A honeycomb structure having channel walls consisting essentially of a
selective catalytic reduction catalyst, wherein the honeycomb
structure:has a cell density of from about 350 to about 600 channels per
square inch of transverse honeycomb cross-section;comprises channel walls
having a wall thickness of about 100 to about 250 microns, said channel
walls occupying at least 20% of the volume of the structure;exhibits a
pressure drop for flowing air not exceeding 110 Pa at a space velocity of
20,000 hr-1, andprovides a catalyst utilization factor of at least
80% and a nitrogen oxide conversion efficiency of at least 45% when
processing a gas mixture comprising 500 ppm (volume) of ammonia and 500
ppm (volume) of nitrogen oxide (NO) at a gas mixture temperature of
250.degree. C. and a space velocity of 20,000 hr.sup.-1.

Description:

BACKGROUND

[0001]The disclosed catalysts and methods relate to the reduction of
nitrogen oxides generated during high temperature combustion processes,
particularly including the treatment of the NOx-containing exhaust
streams from mobile emissions sources such as motor vehicles.

[0010]The efficiencies of honeycomb catalyst structures employed for the
reduction of nitrogen oxides in SCR reactions are governed by a number of
factors, including the composition or reactivity of the catalyst, the
loading of catalyst into the structure, the geometry and microstructure
of the honeycomb, and the upstream exhaust flow spatial conditions
including the composition, temperature, and flow distribution of the
exhaust. For any given set of exhaust flow conditions and given catalyst
of predeterined reactivity and microstructure, the conversion efficiency,
pressure drop, and catalyst cost will be determined by honeycomb geometry
(i.e., cell density, channel wall thickness, diameter, and length) and
catalyst loading.

[0011]While conversion efficiencies can be increased for any particular
honeycomb catalyst design by increasing catalyst loading and cell
density, the catalyst costs for the resulting structures can be high and
the levels of catalyst utilization are reduced. Additionally, the
increased pressure drops across the thus-modified structures can incur
unacceptable increases in exhaust system backpressure, and thus cause
problematic reductions in engine power output and fuel economy.

[0012]Nitrogen oxides (NOx) are by-products of the combustion of
carbonaceous fuels in air, and together with unburned hydrocarbons and
carbon monoxide are the targets of government regulations limiting
polluting emissions from motor vehicles. In conventional gasoline
engines, governmental limits are being met through the use of so-called
"three-way" catalysts, generally precious metal catalysts that are
dispersed in catalytic coatings applied to refractory monolithic
(honeycomb) supports contained in automobile catalytic converters.
However, such catalysts are not adequate for the removal of the higher
NOx concentrations that are typically found in diesel and lean-bum
gasoline engine exhausts.

[0013]A different technology, based on the selective catalytic reduction
(SCR) of nitrogen oxides using ammonia as a reductant, has been developed
for the removal of NOx from stack gases emitted by fossil-fuel-fired
power plants. Adapting the SCR process for NOx reduction from
gasoline and diesel engine exhaust gases is a current area of
development.

[0014]Several different catalyst compositions and products have been
proposed for use in SCR processes, including precious metals, base metal
oxides of tungsten, vanadium, and titanium, and zeolite-based materials
including Fe-- and Cu-impregnated zeolites. Product configurations vary
with the application but have included beads, plates and honeycombs.

[0015]Effective SCR systems for mobile emissions control applications must
provide high deNOx performance (desirably a complete conversion of the
NOx compounds present in the exhaust to N2). However low catalyst
loadings are desirable in order to limit system costs. Good mechanical
strength and thermal durability are also needed to enable the catalysts
to survive handling, canning, and vibration and thermal cycling in use.
At the same time, catalyst configurations that can facilitate low exhaust
backpressure are needed to maintain engine efficiency and fuel economy.

[0016]One approach that has been suggested for adapting SCR processes to
the treatment of automobile exhaust gases has involved applying SCR
catalyst coatings to ceramic honeycomb supports such as currently used to
support three-way automobile exhaust catalysts. However, catalyst
coatings provide only modest catalyst loadings when compared with
extruded SCR catalysts, and thus offer only limited conversion
efficiencies, especially at low temperatures or low exhaust gas flow
rates. Higher heat capacities for better thermal shock resistance,
reduced exhaust back-pressures for improved fuel economy, higher
resistance to catalyst loss through spalling of the catalyst coatings,
and reduced unit weight and volumne due to elimination of the inert
ceramic support, are other advantages that could potentially flow from
the use of extruded rather than coated catalysts. Avoiding the process
and supply chain costs associated with the need to employ coating
processes and equipment would also be attractive.

[0017]Unfortunately most present designs for extruded honeycomb SCR
catalysts, including those currently used in power plant stack gas
treatment systems, are not suitable for use in mobile emissions control
applications. Among other shortcomings, such catalysts do not provide the
conversion efficiencies required to meet current and proposed
environmental regulations limiting NOx emissions from diesel and/or lean
bum gasoline engines, particularly at the relatively high exhaust gas
flows typical of such engines. Thus while extensive attention has been
focused on understanding the relationship between catalyst composition
and efficient SCR NOx reduction, SCR catalysts and catalytic treatment
methods employing SCR NOx control that combine a high level of NOx
reduction with low exhaust system pressure drop, low cost, good
mechanical and thermal durability, and a high level of catalyst
utilization to minimize catalyst cost have yet to be provided.

[0018]The catalysts hereinafter disclosed are honeycomb monoliths of solid
SCR catalytic material, formed for example by the extrusion of
plasticized catalyst formulations from honeycomb extrusion dies. A
typical honeycomb 10 as illustrated in FIG. 6 of the accompanying
drawings comprises an array of adjoining parallel channels 12 bounded by
thin interconnecting channel walls or webs 14, the channels being
open-ended and extending from a first or exhaust gas inlet end 16 of the
honeycomb structure to a second or exhaust gas outlet end 18 of the
structure.

[0019]To provide high NOx removal efficiency, the honeycomb structures
incorporate a volume of catalyst sufficient to allow for the diffusion
and reduction of NOx by a suitable reductant at active reduction sites
within the channel walls even at high exhaust gas flow rates. However,
the volume fraction of catalyst is not so large as to include excess
catalytic material that is substantially inaccessible to NOx reactant
diffusion at those flow rates, or that acts to obstruct exhaust flow and
thus increase pressure drop across the honeycomb structure. Thus the
volume fraction of actively functioning catalyst in the structure, i.e.,
the catalyst utilization factor, is high.

[0020]Embodiments of honeycomb catalysts providing these characteristics
of the disclosure include honeycomb structures having channel walls
consisting essentially of selective catalytic reduction catalyst, and
where the channel walls occupy at least 20% of the volume of the
structure. The weight and distribution of the channel walls within the
honeycomb structures are selected such that the structures exhibit a
pressure drop for flowing air not exceeding 110 Pa at a space velocity of
20,000 hr-1, for example at a honeycomb channel length of 15 cm. For
the purposes of this disclosure the terms "selective catalytic reduction
catalyst" and "SCR catalyst" include both pure catalysts and dispersions
of such catalysts in solid matrix materials or fillers that can bind,
support and secure the pure catalysts to or into the walls of the
honeycomb catalysts. Examples of powdered matrix materials that can be
used as fillers or binders for this purpose include alumina, cordierite,
zircon, zirconia, mullite and the like.

[0021]Pressure drops through the honeycomb structures are controlled
principally through appropriate selections of channel wall thickness,
honeycomb cell density, and channel length. Honeycomb cell densities are
defined in terms of the number of honeycomb channels per unit of
honeycomb cross-sectional area as measured in a plane perpendicular to
the direction of channel orientation in the honeycomb in accordance with
standard practice. Specific embodiments of the disclosed catalysts have
channel wall thicknesses not exceeding 250 microns, such thicknesses
being effective to maintain high catalyst utilization factors even at gas
flow rates typical of motor vehicle exhaust systems. Thus the disclosure
includes embodiments of the above-described catalysts that provide a
nitrogen oxide conversion efficiency of at least 45% when processing a
combustion exhaust gas mixture comprising 500 ppm (volume) of ammonia and
500 ppm (volume) of nitrogen oxide (NO) at a gas mixture or reaction
temperature of 250° C. and a space velocity of 20,000 hr-1.

[0022]The disclosure additionally includes methods for treating gas
streams comprising nitrogen oxide pollutants utilizing the disclosed
honeycomb catalyst structures. Embodiments of those methods include a
method for treating a gas stream to remove nitrogen oxides therefrom
comprising the steps of introducing a nitrogen oxide reductant into the
gas stream, and passing the gas stream having the reductant through a
honeycomb structure having channel walls consisting essentially of a
selective catalytic reduction catalyst as herein described. The selective
catalytic reduction catalyst used in the practice of the disclosed
methods occupies at least 20% of the volume of the structure and the
structure exhibits a pressure drop for flowing gas (e.g., room
temperature air) not exceeding 110 Pa at a space velocity of 20,000
hr-1, for example at honeycomb channel lengths of up to 15 cm.

[0023]The disclosed concepts can be applicable to a wide variety of SCR
catalysts and NOx exhaust stream conditions. However, they can be
particularly applied to the design of extruded honeycomb catalysts of or
zeolytic or molecular sieve composition. Zeolites and other such
catalysts can be adapted for use in methods to treat combustion engine
exhaust gases. The disclosed concepts can be applied to the selection of
honeycomb monolith cell densities and channel wall thicknesses for
extruded flow-through honeycomb catalysts, which cell densities and wall
thicknesses can deliver high-level deNOx performance with ammonia-based
reductants, at low pressure drops, and reduced catalyst costs. Thus the
following descriptions and examples refer particularly to such catalysts
and methods even though the concepts involved are not limited thereto.

[0024]Selective catalytic reduction (SCR) processes are known to involve
the catalytic reduction of nitrogen oxides with ammonia or an ammonia
source in the presence of atmospheric oxygen, to produce nitrogen and
steam. The following reactions are illustrative:

4 NO+4 NH3+O2→4N2+6 H2O

2 NO2+4 NH3+O2→3N2+6 H2O

NO+NO2+2NH3→2N2+3 H2O

[0025]In extruded honeycomb catalysts, these SCR reactions occur within
the porous channel walls or webs of the monolithic structure. Thus after
overcoming mass transfer limitations affecting the transfer of reactant
gases from the flowing gas stream to the channel walls, the gases must
then diffuse from the outer wall surfaces through and into the interiors
of the pores in order to reach active catalyst sites. Then, once reaction
occurs, the reaction products must traverse the reverse path while
overcoming similar mass transfer resistance. The performance of any
monolithic catalytic structure is therefore limited by the extent to
which the reacting gases can reach active catalyst sites via pore
diffusion. In the foregoing the heavier catalyst loadings can involve
longer diffusion path lengths for gaseous reactants, and therefore the
SCR conversion improvements resulting from higher catalyst loadings will
not necessarily be in proportion to the amounts of catalyst added. This
effect can be numerically represented by a value referred to herein as a
catalyst utilization factor. A useful catalyst utilization factor can be
calculated from an expression such as:

wherein SCR performance is measured in terms of percent of NOx conversion
under specified exhaust gas inlet conditions. The denominator in the
above expression refers to catalyst performance under a hypothetical
situation where the gaseous reactant gases come in contact with all of
the reactive sites in the catalyst as soon as the gases touch the channel
wall. That performance can be calculated utilizing known honeycomb
catalyst modeling tools such as by "turning off" pore diffusion
resistance in the models, for example by making pore diffusion infinitely
fast.

[0027]The DETCHEM software includes a module for modeling flow-through
substrates incorporating catalyzed washcoat layers. Adapting that module
to the modeling of the disclosed extruded solid honeycomb catalysts
involves treating the channel walls of the honeycomb as an "apparent
washcoat", with the distribution of catalyst across the thickness of
those channel walls assumed to be uniform. The original and adapted
models both assume identical conditions within each channel of the
honeycomb structures, with negligible axial dispersion.

[0028]The kinetics for the above NOx reduction reactions as determined
from bench tests of honeycomb structures extruded from selected SCR
catalysts can be factored into the equations to produce a fully
two-dimensional transient two-phase mathematical model of an SCR
honeycomb monolith reactor. No further adjustments are required for the
model to accurately project honeycomb catalyst performance over a
relatively wide range of catalyst loadings, honeycomb geometries, and gas
flow rates.

[0029]FIG. 1 compares representative bench test conversion data with
projected (modeled) conversion performance for two extruded zeolite
honeycomb catalysts of differing honeycomb geometry after hydrothermal
aging. Conversion efficiencies are reported as percent conversions of NOx
present in a synthetic exhaust gas stream, reported on the y-axis, over a
range of honeycomb inlet temperatures from 150° C. to 450°
C. reported on the x-axis. The two honeycomb geometries for the catalyst
designs evaluated in FIG. 1 include a first geometry (Curves M and M')
having a channel wall thickness of 0.010 inches, and a second geometry
(Curves N and N') having a channel wall thickness of 0.006 inches. Both
geometries were of cell densities of 400 channels/in2 of honeycomb
cross-section.

[0030]The synthetic exhaust gas used for testing and modeling comprises
500 ppm (volume) of nitrogen oxide (NO) and 500 ppm (volume) of ammonia
in air, that mixture being passed through the honeycomb catalysts at
actual or modeled space velocities of 20,000 hr-1. Each of the
extruded honeycomb catalyst designs evaluated consists of a cylindrical
shape 2.5 cm in diameter by 2.5 cm in length with the honeycomb channels
running parallel with the cylinder length.

[0031]The modeled conversion results for each of the two honeycomb
geometries evaluated in FIG. 1 are represented by dashed curves M' and
N', while the bench test results are represented by solid curves M and N.
The data thus presented clearly confirm the validity of the adapted
models, in that the modeled conversion results conform closely to the
bench test results for both of the honeycomb geometries evaluated.

[0032]Further validation of the adapted models is provided by tests
designed to track honeycomb pressure drops as a function of gas flow rate
through the honeycombs. FIG. 2 of the drawings plots modeled and bench
test data for two honeycomb catalyst designs having the same cell density
but different wall thicknesses. The honeycomb samples evaluated are of
the same exterior dimensions and channel orientation as the honeycombs
characterized in FIG. 1 of the drawings. The first design, characterized
by Curves R and R' in FIG. 2, has a channel wall thickness of 0.004
inches, while the second design, characterized by Curves S and S', has a
channel wall thickness of 0.010 inches. Both of the evaluated designs
have cell densities of 400 cells/in2 of honeycomb cross-sectional
area as measured transverse to the direction of channel orientation.

[0033]The measured and calculated pressure drops for the honeycomb
catalysts evaluated in FIG. 2 are reported in inches of water on the
y-axis, while gas flow rates for the catalysts are reported in cubic feet
per minute on the x-axis. FIG. 2 demonstrates a good correspondence
between the bench test results for the two designs, indicated by the
solid lines R and S, and the modeled results, indicated respectively by
the broken lines R' and S'. Thus these data further confirm the value of
the adapted models as useful tools for projecting the performance of
honeycomb SCR catalysts over a wide range of geometric design parameters.

[0034]For honeycomb catalysts having channel walls formed entirely or
substantially entirely of catalyst-bearing material, higher deNOx
performance is generally associated with either increased catalyst
content, e.g., higher catalyst concentrations per unit volume of
honeycomb catalyst, which are expensive in terms of catalyst cost, or
with higher pressure drops, which are expensive in terms of higher fuel
consumption penalties. The data presented in FIGS. 1 and 2 of the
drawings illustrate these effects. Thus the honeycomb catalyst of 400/10
(cell density/wall thickness) design (Curves M and M' in FIG. 1), with a
catalyst content of 36% by volume, exhibits higher conversion efficiency
at equivalent inlet temperatures than the 400/6 design (Curves N and N'),
with a catalyst content of 25% by volume. On the other hand, the
honeycomb catalyst design of FIG. 2 having the higher channel wall
thickness (the 400/10 honeycomb design of Curves R and R') exhibits
substantially higher pressure drops at equivalent gas flow rates than the
400/4 design of Curves S and S'.

[0035]A further disadvantage of increased catalyst loading in solid SCR
catalysts is that, due to gas diffusion limitations such as discussed
above, the level of catalyst utilization decreases with increasing
catalyst or channel wall thickness even though some increases in
conversion efficiency may be realized. These competing effects are
illustrated by the NOx conversion and catalyst utilization data reported
in FIG. 3 of the drawings.

[0036]The catalyst samples analyzed to provide the data plotted in FIG. 3
fall into five separate families A through E, each family comprising one
or more samples of the same cell density but differing channel wall
thickness. Each of the solid curves labeled A through E in FIG. 3 plot
conversion results for one family in percent of NO conversion on the
y-axis as a function of channel wall or web thickness on the x-axis. Each
of the broken line curves labeled A' through E' plots catalyst
utilization factors (in percent utilization) on the y-axis as a function
of channel wall (web) thickness on the x-axis for the same families of
catalyst samples. The conversion percentages reported in FIG. 3 are for a
synthetic exhaust gas having the composition, space velocity, and
temperature of the exhaust gas used to generate the model and bench test
conversion data shown in FIG. 1 of the drawings. Table I below reports
the cell densities of each of the five families characterized in FIG. 3

[0037]As the modeled catalyst utilization data in FIG. 3 suggest, the
degrees of catalyst utilization in these honeycomb catalyst designs
(broken line curves) are found to decrease with increasing channel wall
thickness for each of the series evaluated. As expected the catalyst
utilization values are found to be substantially independent of honeycomb
catalyst cell density. While other factors must also be taken into
account in designing a honeycomb catalyst suited for the control of
engine exhaust emissions, the value of maintaining a high degree of
catalyst utilization to control catalyst cost is evident from these data.

[0038]The discovery of honeycomb catalyst configurations of high NOx
conversion efficiency, but with the controlled catalyst loadings and
limited pressure drops required for economic diesel and lean burn engine
NOx emissions control, has required further studies of catalyst
performance data involving novel indices of catalyst performance. The
first such performance index, referred to as a conversion/loading index
(C/L Index), corresponds to a ratio of NOx conversion level to catalyst
loading for each of a number of selected honeycomb catalyst design to be
evaluated. That index provides a basis for comparing those designs over a
range of catalyst loading levels and corresponding conversion levels to
identify designs offering higher than expected conversion activity for a
given level of catalyst loading.

[0039]FIG. 4 plots modeled NOx conversion activity (y-axis) for five
families of honeycomb catalyst design over a broad range of
conversion/loading (C/L Index) values (x-axis). Broken line curves
labeled A-E connect data points within each family comprising multiple
evaluation samples; the cell densities are invariant within each family,
and correspond to the densities reported for honeycomb designs A-E in
Table I above. The C/L index values increase from left to right along the
x-axis of the graph, reflecting decreasing channel wall thicknesses, and
thus decreasing catalyst loadings, in that direction on the graph. All
NOx conversion values in Table 4 are calculated for a synthetic
exhaust gas composition, space velocity, and gas processing temperature
as described above in connection the generation of the data illustrated
in FIG. 1.

[0040]As the curves in FIG. 4 suggest, there are substantial differences
in the levels of NOx conversion observed among honeycomb catalyst designs
having equivalent conversion/loading indices. Designs that exhibit higher
levels of NOx removal will be of primary interest for further
development. However, evaluating competing designs in terms of the C/L
Index can also be helpful in identifying honeycomb designs that provide
only marginal NOx conversion levels (e.g., conversions below 45% of NO at
a 250° C. inlet temperature) even at high catalyst loadings. The
100 cpsi designs plotted in FIG. 4 are examples of the latter designs.

[0041]The second performance index of interest for evaluating honeycomb
catalyst designs, termed a conversion/loading/pressure drop (C/L/dP)
index, adds a pressure drop dimension to the above C/L evaluation
analysis. That index, consisting of a ratio of conversion level to
catalyst loading to pressure drop for each of the evaluated designs,
provides an approach for comparing designs of similar catalyst loading
(and therefore roughly equivalent catalyst cost) to identify design
solutions offering higher conversion efficiencies yet lower pressure
drops at a given loading level.

[0042]FIG. 5 of the drawings plots modeled NOx conversions (y-axis) for a
number of different honeycomb catalyst designs over a range of C/L/dP
Index values (x-axis). The honeycomb designs evaluated comprise the same
five families of catalyst design A-E reported in Table I above and
characterized in FIG. 4 above, with the broken line curves connecting
data points within each family in FIG. 4 again being correspondingly
labeled. All NOx conversion values are again calculated for a
synthetic exhaust gas composition, space velocity, and gas processing
temperature equivalent to that described above in connection with the
data reported in FIGS. 1.

[0043]As indicated in FIG. 5, the C/L/dP Index values increase from left
to right on the x-axis, being dominated by decreases in catalyst loading
resulting from decreases in channel wall thickness in that direction. For
the overall C/L/dP Index, however, the increases in index value are
moderated by the changing pressure drop (dP) values, these also
decreasing from left to right as a consequence of the reductions in
channel wall thickness.

[0044]Catalyst cost considerations alone could suggest the selection of
catalysts with higher C/L/dP indices from this design space, but NOx
conversion requirements will limit the number of satisfactory design
choices to those of somewhat lower C/L/dP Index, i.e., of higher catalyst
loading. Advantageously, from among the latter choices, the data permit
the identification of designs with higher conversion activity and lower
pressure drop that will still meet a selected required minimum NOx
conversion level. Thus the data permit the design of new honeycomb
catalyst configurations that correctly balance the competing
considerations of catalyst cost, honeycomb pressure drop, and NOx
conversion effectiveness.

[0045]The disclosed catalysts and catalyst methods include embodiments
wherein the honeycomb structure includes a selective catalytic reduction
catalyst of zeolitic or molecular sieve structure. Specific examples
include those where the catalyst can be selected from the group
consisting of beta zeolite, ZSM-5 zeolite, mordenite,
silico-aluminophosphates, metal-impregnated zeolites including, for
example, copper- or iron-zeolites, and combinations thereof. These and
similar zeolitic catalysts can be used to make embodiments of honeycomb
catalyst structures which, when processing a gas mixture comprising a
combination of 500 ppm (volume) of ammonia and 500 ppm (volume) of
nitrogen oxide (NO) in air at at a space velocity of 20,000 hr-1 and
a gas temperature of 250° C. at the catalyst inlet surface,
provide a nitrogen oxide conversion efficiency of at least 45% within a
honeycomb channel length of 15 cm. For the purposes of the disclosure,
effective NOx conversions will extend to conversions of any of
nitric oxide (NO2), nitrogen oxide (NO), nitrous oxide (N2O),
and mixtures thereof, provided only that the gas mixture includes
stoichiometrically sufficient proportions of ammonia or an ammonia
source, such as urea, to substantially complete the reductions.

[0046]Further embodiments of the disclosed catalysts include honeycomb
catalyst structures having a honeycomb channel length of at least 15 cm,
as well as honeycomb catalyst structures having catalyst utilization
factors of at least 80%. Structures having channel walls of a thickness
not exceeding about 250 microns as described above can readily meet this
high catalyst utilization level if the walls are sufficiently porous to
be gas-permeable.

[0047]As noted above, the discovery of honeycomb catalyst structures
having design parameters offering high conversion efficiencies in
combination with moderate pressure drop and reasonable catalyst cost has
been enabled by analyses of conversion data including performance index
curves such disclosed in FIGS. 4 and 5. Particular embodiments of
catalysts developed from such analyses generally include honeycomb
catalyst structures having a cell density of at least 350 channels per
square inch of transverse honeycomb cross-section, e.g., from about 350
to as many as 600 channels per square inch of a transverse honeycomb
cross-section, and with channel wall thicknesses not exceeding about 250
mircrons, e.g., from 100-250 microns. Again, the honeycomb catalyst
structure may be formed entirely of an SCR catalyst, but more typically
will be a structure comprising the selective catalytic reduction catalyst
distributed within the channel walls of the structure in a supporting
matrix of a material, such as cordierite or alumina, that is typically
catalytically inert or substantially inert with respect to nitrogen oxide
conversion.

[0048]Embodiments of the above-described catalysts can readily meet the
prescribed pressure drop and NO conversion characteristics, for example
in unitary structures of 15 cm channel length or greater. However, where
the properties of the selected SCR catalyst are such as to favor
honeycomb catalyst manufacture in segments of shorter length, suitable
honeycomb catalyst structures can be composite structures of whatever
lengths are required for the particular application of interest. An
example of such a structure is one made up of a stack of channel-aligned
honeycomb slices providing a combined channel length of the selected
magnitude. References to honeycomb catalyst structures in the disclosure
are thus intended to include such composite catalyst structures where the
selected channel lengths require it.

[0049]Methods for treating gas streams to remove nitrogen oxides in accord
with the disclosure include those wherein the gas stream is a combustion
exhaust gas such as produced by a fossil-fuel powered rotary, turbine or
piston engine, and where the nitrogen oxides in the exhaust gas include
at least one of NO and NO2. Embodiments of such methods particularly
include those wherein the reductant for nitrogen oxide removal in
accordance with SCR processing is ammonia, or an ammonia source such as
urea. Again, embodiments of the disclosed methods wherein the catalyst
comprises a zeolite or zeolitic or molecular sieve material, for example
where the catalyst is selected from the group consisting of beta zeolite,
ZSM-5 zeolite, mordenite, silico-aluminophosphate, metal-impregnated
zeolite including Fe-zeolite or Cu-zeolite, and combinations thereof, are
highly effective.

[0050]In general, the disclosed methods will most frequently be practiced
in embodiments where the reductant is introduced into and present in the
exhaust stream in a proportion at least stoichiometrically sufficient
convert the nitrogen oxides in the exhaust stream to nitrogen and water.
Such embodiments include those where the exhaust gas stream is introduced
into the honeycomb catalyst structure at a flow rate and temperature
sufficient to achieve the reduction and removal of at least 45% of the
nitrogen oxides in the exhaust at catalyst inlet temperatures of
250° C. and above. For reasons of economy, including catalyst cost
control and honeycomb catalyst pressure drop reduction, embodiments of
the disclosed methods will include those wherein the channel walls of the
selected honeycomb catalyst have a thickness sufficiently reduced to
provide a catalyst utilization factor of at least 80%.

[0051]The following illustrative example describes the production and use
of a representative honeycomb catalyst structure in accordance with the
disclosure.

EXAMPLE

[0052]A honeycomb SCR catalyst is manufactured from a metal-impregnated
ZSM-5 zeolite powder. To prepare the zeolite powder, a saturated aqueous
solution of ferrous gluconate comprising about 10% ferrous gluconate and
the remainder water by weight is provided. A commercially available ZSM-5
zeolite powder is then added to the solution to produce a thin zeolite
slurry comprising zeolite and gluconate solution in a ratio of 1:1 by
weight. The slurry is then spray-dried to produce an iron-zeolite powder.

[0053]A plasticized mixture comprising the iron-zeolite powder is next
prepared for forming into an extruded honeycomb catalyst. A blended
powder mixture is first produced by combining the spray-dried
iron-zeolite powder with a powdered alumina matrix material in a
proportion of 40 parts iron-zeolite to 60 parts alumina by weight. The
alumina matrix material is a calcined Alcoa® A-16 alumina powder.

[0054]An aqueous silicone emulsion to serve as a liquid vehicle and
permanent binder is then added to the powder mixture along with a
quantity of a methyl cellulose powder to serve as a temporary binder,
with the resulting mixture then being worked into a plastic mass. The
amount of silicone emulsion added is sufficient to plasticize the powder
mixture, and the amount of methyl cellulose added is sufficient to permit
the plasticized material to maintain shape integrity upon drying.

[0055]The plasticized mixture thus provided is next extruded through a
honeycomb extrusion die to form a wet honeycomb shape, and the wet shape
is air-dried in an oven to produce a dried green honeycomb preform. The
honeycomb preform thus provided is then calcined at 850° C. to
produce a strong honeycomb catalyst structure. The cell density and slot
discharge slot width of the honeycomb extrusion die are selected such
that the extruded honeycomb catalyst structure has a cell density of 400
cells/in2 and a channel wall thickness of 0.006 in. (150 microns)
following drying and calcining.

[0056]Testing of the honeycomb catalyst thus provided is carried out
utilizing a synthetic exhaust gas comprising an air stream containing 500
parts per million (volume) of nitrogen oxide (NO) and 500 parts per
million (volume) of ammonia. Small honeycomb catalyst samples of
cylindrical shape, each approximately 2.5 cm in diameter and 2.5 cm in
length with the honeycomb channels running parallel with the cylinder
length, are cut from the extruded honeycomb catalyst structure for
testing. Testing involves passing the synthetic exhaust gas through the
honeycomb samples at a space velocity of 20,000 hr-1 while raising
the temperature of the gas as measured at the honeycomb inlet surface
from 150° C. to 450° C. In the course of this testing the
catalyst samples are found to convert in excess of 50% of the available
NO and ammonia to nitrogen and water at a gas inlet temperature of
250° C. and more than 90% of the NO and ammonia to nitrogen and
water at a gas inlet temperature of 310° C.

[0057]Table II below summarizes honeycomb SCR catalyst performance data
for various honeycomb SCR catalyst designs of similar catalyst
composition under modeled conversion testing conditions such as above
described. Catalyst embodiments within the scope of the disclosure, as
well as comparative embodiments that exhibit performance or cost problems
such as excessive pressure drops, low NO conversion efficiencies, and/or
low levels of catalyst utilization, are illustrated. Included in Table II
for each of the honeycomb catalyst designs evaluated are values for
honeycomb cell density, honeycomb channel wall thickness, honeycomb
pressure drop, nitrogen oxide (NO) conversion efficiency, and catalyst
utilization factor.

[0058]The data in Table II are representative of the characteristics of
honeycomb SCR catalysts of approximately 15 cm diameter and 15 cm channel
length. The pressure drop values for the catalysts are calculated at an
airflow rate yielding a space velocity of 20,000 hr-1 through the
honeycombs. The catalytic conversion efficiencies are for the case of a
synthetic exhaust gas comprising 5.00 ppm (volume) each of NH3 and
NO, that gas passing through the catalysts at the 20,000 hr-1 space
velocity and at a gas temperature of 250° C. as measured at the
catalyst inlet surface.

[0059]As the data in Table II reflect, at channel wall thicknesses above
about 10 mils, NO conversion efficiency can be high but catalyst
utilization can fall below 80%, resulting in excessive catalyst cost.
Comparative example 4C is illustrative. On the other hand, at cell
densities substantially below 400 cells/in2, e.g., below 350
cells/in2, achieving 45% NO conversion levels at 250° C. and
at space velocities of 20,000 hr-1 within channel lengths of 15 cm
can be difficult unless channel wall thicknesses are high. Comparative
example 6C, for example, achieves adequate NO conversions, but at a
catalyst utilization of only 70%. Finally, catalyst designs featuring
high cell densities, such as comparative example C1, will typically
exhibit excessive pressure drops, while reducing cell densities to reduce
pressure drops, as in comparative example C2, can result in inadequate
conversion efficiencies.

[0060]Based on analyses of competing designs from data such as presented
in the drawings and in Table II above, honeycomb SCR catalysts comprising
a selective catalytic reduction catalyst distributed within the channel
walls of the structure and offering the combined advantages of high
conversion efficiency, low pressure drop, and a high level of catalyst
utilization can be provided within a cell density range of 350-600
cells/in2 and a channel wall thickness range of 100-250 microns.
Within those ranges, lower pressure drops in combination with higher
conversion efficiencies can then be realized through the selection of
lower channel wall thicknesses where higher cell densities are to be
employed.

[0061]From the foregoing descriptions and examples it is apparent that the
disclosed principles of SCR honeycomb catalyst design and use are
applicable to a broader range of catalysts and applications, and may be
readily extended to other honeycomb monoliths of solid catalyst
construction to insure high catalyst utilization, increased catalytic
efficiency, and reduced catalyst cost. Thus a variety of modifications
and adaptations of the particular catalysts and methods disclosed herein
may be utilized by those of ordinary skill in the art without departing
from the spirit and scope of the appended claims.